Underwater transducer element



Sept. 22, 1964 s. HANlSH UNDERWATER TRANSDUCER ELEMENT 2 Sheets-Sheet 1 Filed NOV. 50, 1959 INVENTOR SAM H AN \SH ATTORNEY UNDERWATER TRANSDUCER ELEMENT Filed NOV. 30, 1959 2 Sheets-Sheet 2 INVENTOR SAM H AN l S H E y/MAW ATTORNEY United States Patent 3,150,347 UNDERWATER TRANSDUCER ELEMEN Sam Hanish, Glassmanor, Md, assignor, by mesne assignments, to the United States of America as represented by the Secretary at the Navy Filed Nov. .30, 1959, Ser. No. 856,320 9 Claims. (Cl. 349-) This invention relates in general to electromechanical transducers and, in particular, to such transducers which employ flexure disc elements.

Electromechanical transducers of the type employed in underwater sound applications generally comprise an array of individual elements aligned in a plane or otherwise cooperatively disposed. A wide variety of transducer elements is available for use in such arrays, and these elements have selected features and advantages which have been found useful in one or more distinct underwater sound applications. In general, the prior art devices have involved one or more controlling parameters and the number of parameters has determined the extent of their adaptability to other applications than the one for which the device was specifically designed. For example, transducers employing flexure disc elements have been complicated, and their application limited, by a non-vibrating countermass consideration which necessitates an increase in mass as the operating frequency is reduced.

It has long been recognized that a high-power, resonant transducer capable of bilateral acoustic radiation at low frequencies is needed and would be welcomed as a substantial advancement of the art.

Accordingly, it is an object of this invention to provide a high-power, resonant, low-frequency transducer element.

It is another object of this invention to provide an improved transducer element capable of bilateral acoustic radiation.

It is a further object of this invention to provide an improved transducer element which is adaptable to a wide variety of underwater sound applications.

It is also an object of this invention to provide a tran ducer element which involves no significant countermass consideration.

It is another object of this invention to provide a transducer element which affords low acoustic impedance matching.

It is still another object of this invention to provide a low-frequency transducer element which may be sturdily constructed.

It is a further object of this invention to provide an improved transducer element which may be easily manufactured.

Other objects of this invention will become apparent upon a more comprehensive understanding of the invention for which reference is bad to the following specification and drawings, wherein similar structural parts are similarly identified in each of the figures.

FIG. 1 depicts an underwater projector which is cut away to show elements of the projector in their normal arrangement.

FIG. 2 shows a single transducer element in accordance with one embodiment of the present invention.

FIG. 3 depicts a single transducer element in accordance with a second embodiment of this invention.

FIG. 4 is a cross-sectional view of a flexure disc which might be substituted for the flexure discs shown in the embodiments of FIGS. 2 and 3.

Briefly, the invention provides an improved transducer element for use in underwater sound applications by a unique utilization of the advantages of bender resonant radiation. The element comprises 'a pair of bender discs disposed to operate in bender resonance. The bender discs in each transducer element are mounted in opposiice tion on a central pin to provide a high etficient bilateral response. In addition, a pressure control feature is incorporated to permit the use of the elements at extreme depths.

Referring now to the drawings:

FIG. 1 depicts an underwater projector which includes a plurality of of the transducer elements of this invention in a typical cooperative assembly, generally referred to as an array or bank. In FIG. 1 the separate elements are indicated at 11 within the partially cut away housing 12. It will be appreciated that the relative size of the individual elements and the housing is disproportionate in the drawing for purposes of clarity. In an operational assembly the housing might encompass an array approximately eight feet in diameter while the individual elements in the array might have a diameter of four inches or less. Arrays of 40 elements have been constructed in the geometric shape of a plain hexagon with the elements spaced equidistant, that is with a selected constant center to center dimension between adjacent elements. It will be appreciated that such an arrangement affords equal loading on the individual elements, and that equal loading of the elements is a critical requirement in some applications.

PEG. 2 depicts one embodiment of the transducer element of this invention in a detailed cross-sectional showing thereof. In the drawing, two fieXure disc elements, each comprising a metallic backing plate 21 and a ceramic disc 22, are centrally supported by means of the pins 23 and 24 respectively on opposite sides of the bafile plate assembly 25. it will be appreciated that each of the pins 23 and 24 which are shown as separate elements in the embodiment of FIG. 2 may be incorporated in an alternate arrangement as an integral part of, for example, the metallic backing plate 21 or, in another alternate arrang ment, as an integral part of the bafiie plate assembly 25. In either of the last two embodiments, of course, only one end of the pins need be threaded for purposes of a simple assembly and disassembly.

The baffle plate assembly 25 is symmetrically disposed at the center of the transducer element enclosure and in the embodiment of FIG. 2 comprises two perforated discs mounted back to back on a central coupling shaft indicated at 26. In this embodiment, the coupling shaft is hollowed on either end and the hollow has internal threads to receive the pins 23 and 24.

The transducer element enclosure shown in FIG. 2 is a sealed unit suitable for direct immersion in the liquid medium and comprises a tubular side section 27 and two end sections which complete the enclosure. In this particular embodiment the end sections of the enclosure each comprise a rubber acoustic window indicated at 28 which is held in place by a window flange indicated at 29. The type of baffle plate assembly shown in the drawing and the support means therefor are not, of course, critical to the invention and may be replaced by any suitable means for limiting the travel of the outer edge of the flexure disc-s.

It will be noted that the perforated discs of the battle plate assembly are widely separated one from the other and are supported within the enclosure by means of peripheral connection to the tubular side section 2.7.

The flexure discs in the embodiment of FIG. 2 are coupled electromechanically, in the radial mode, but receive their fiexural excitation through the bilamellate principle. That is, the backing plate 21 and the ceramic disc 22 produce the desired ilexure in a manner analogous to the effect produced by two flexure plates oppositely polarized, glued together and driven electrically in parallel such that as one plate contracts (compression state), the other plate expands (tension state). It will be appreciated that while the result is similar, the structural rearrangement in the present invention, which involves a reduction in the number of active elements, one inactive disc and one active disc in place of two active discs, reduces the magnitude of the total electromechanical coupling. It has been found that this reduction in magnitude does not seriously affect the advantageous utilization of the bilamellate principle.

The number of modes hat can be excited in a flexural disc are infinite. In uniform radial excitation, however, the displacements of the neutral plane of bending of the flexing disc will be in-phase effectively for only one mode, all other modes exciting out-of-phase displacements. When all active surface areas execute their amplitude of motion in phase, the disc is considered to be in bender resonance. Disc plates designed for acoustic transmission in liquid media should exhibit in-phase displacement over the entire active area, except where special mountings allow selection of portions of the active phase to see the medium. In all cases, for optimum loading, the transducer should be mechanically resonant, the resonant frequency being determined by the dimensions of the plate, the external restraints on the plate, and the mechanical properties of the liquid media.

In a typical embodiment wherein the disc plates are supported at the center as shown in FIG. 2 and the dimensions are determined for use in a Water liquid media at a frequency of 2 k0,, the fiexure discs might be as follows:

In the selection of the metal material for the backing plate 21 the mechanical characteristics of the ceramic disc 22 must be considered. It has been found that brass (SAE 72 /2) is an excellent compromise material for a ceramic-metal bilamellate transducer. This stems from similar mechanical properties (Youngs Modulus, velocity of compressional waves, etc.) and from the good machinability of this common metal. It will be appreciated, however, that materials other than brass may be employed if desired; for example, other metals with lower moduli and densities afford savings in weight with the added advantage of a lower mechanical Q in water. The use of aluminum alloys is especially desirable in this regard. The pins indicated as 23 and 24 in the drawing, which support the metallic backing plate, should be of such diameter that the vibration is not seriously affected thereby. A good choice for the diameter of the pins 23 and 24 has been found to be the radius of the backing plate or less.

In the selection of the ceramic disc, the piezo ceramic properties of the material are particularly critical to the operation of the transducer.

It has been found that materials and construction having high values of k Q where k is the effective coelficient of electromechanical coupling and Q is the mechanical Q, afford maximum acoustic power.

In addition, it is desirable to have a ceramic body with a high dielectric constant so that the alternating current impedance, measured across the ceramic at low frequencies will not be excessive. A magnitude of dielectric constant that has proved useful is K equals 1000 to 1700.

Piezo ceramic bodies having the properties noted above are serviceable for low duty cycles. In all high-power continuous duty applications, however, it is further necessary to guard against depolarization due to waste heat generation. A conventional piezo ceramic body (96% BaTiO plus 4% CaTiO operating on remanence, begins to depolarize at degrees centigrade. Bodies with other constituents (lead niobate, lead zirconate, lead tantalate, etc.) can have higher transition points, and will not depolarize at twice or three times the limit temperature of a conventional body. Where available, these bodies may be used, with the understanding that higher drive voltages may be needed to obtain the same power output as with the conventional body. This is due to the fact that higher transition temperatures are sometimes obtained at the expense of lowered dielectric constants. Lead zirconate or lead tantalate piezoelectric ceramics may be selected for their recognized very high piezo electric activity. It has been found that these ceramics provide higher values of K Q for identical constructions than barium calcium titanate, for example.

Ceramic thickness relative to metal thickness determines the coefiicient of electromechanical coupling. An optimum arrangement is one in which the cement bond between the ceramic and the metal does not sense the maximum sheer stress due to fiexure. This can be obtained by making the ceramic thickness less than the metal thickness, at the expense, however, of electromechanical coupling. A good selection is to make the ceramic thickness 30% to 40% of the total thickness of the bender disc element.

FIG. 3 depicts a second embodiment of the transducer element of this invention in a detailed cross-sectional showing thereof. This embodiment is substantially similar in structure and operation to the embodiment of FIG. 2 but includes pressure compensation means which enables use of the transducer at greater depths.

In FIG. 3, a watertight internal bellows assembly 30 is shown disposed within the bafiie plate assembly 25 and around the coupling shaft 26 with the direction of expansion indicated by an arrow. The bellows assembly 30 is shown connected to the tubular side section 27 at point 31. The tubular side section 27 is perforated at the point of contact 31 to permit entry of water into the pressure compensating bellows assembly 30. The perforations consist of a great multitude of relatively small bore holes rather than one or two holes of equal area. It has been found that at 2 kc. operating frequency the large number of small holes greatly reduces the air bubble interference to incoming radiation. It will be appreciated, of course, that the bellows assembly 30 may be connected to the exterior of the transducer element by any suitable means and that it is not essential that the bellows assembly be connected to the side section as exemplarily shown in the drawing.

For example, in the bank arrangement shown in FIG. 1 the internal bellows assembly of each transducer might be interconnected by a conventional pressure manifold system (not shown) to the exterior of the housing 12. Alternately, in the single element case, the internal bellows assembly might be connected along the entire circumference of the side section rather than at one point as shown in FIG. 3.

The structure shown in FIG. 3 can provide less than 2 p.s.i. pressure difference across the bender disc element and has been tested to depths as great as 225 feet without detrimental effect on the transducer or operation. It will be appreciated that other types of pressure compensation means than the one shown in FIG. 3 may be employed in this invention and that it is not essential that the pressure compensation means be confined within the housing.

In the embodiments of FIGS. 2 and 3, the ceramic disc 22 is silvered on both sides and the electrical signal is applied or taken across the thickness of the disc via the leads 32 and 33. In a typical application, one side, generally the side nearest the backing plate 21, is grounded and the other side is above ground potential. As shown in FIGS. 2 and 3, the leads from each of the ceramic discs in each element are normally connected in parallel. In the case of the array shown in FIG. 1, the output of each element may be connected in parallel with the out put of the rest. It is understood, of course, that neither the discs nor the elements have to beelectrically connected in parallel. For example, a different magnitude of signal may be applied to selected elements in an array in amplitude shading applications. In such applications, of course, the'selected elements would be fed independently of the rest.

FIG. 4 depicts the bender disc element of this invention in an alternate embodiment thereof which may be directly substituted for the bender disc elements shown in FIGS. 2 and 3. In FIG. 4 the bender disc element comprises two oppositely polarized ceramic discs 41 and 42 and a metallic backing plate 43. The disc element is supported by the pin 23 on coupling shaft 26 in the'manner shown in FIGS. 2 and 3.

The assembly shown in FIG. 4 approaches the true bilaminate condition and thus affords an increased electromechanical coupling. That is, in this embodiment the disc element is driven such that upon excitation one piece contracts and the other expands.

It will be noted that the thickness of each of the two ceramic discs 41 and 42 and the metallic backing plate 43 is not the same. Best results have been obtained with the ceramic disc 41 /2 the total thickness and the metallic these devices as shown.

The single central support for the bender discs which is shown in the drawings provides a parabolic distribution of velocity over the surface of the bender plate and "at the same time maintains a rigid structure. As a COHSfl quence, the single central support is considered preferable for general applications of the device. However, it is to be understood that a central support is not essential toithis invention and that the scope of this invention is not limited to devices involving a single centralsupport.

For example, several supports disposed at selected nodal points on the disc might be employed in cases where the distribution of velocity need not be parabolic.

It will also be appreciated thatthe rubber acoustic Window 28 in FIGS. 2 and 3 maybe replaced by a less lossy material, if desired. For example, the rubber window 28 may be replaced by a membrane of plastic, steel or aluminum. j

Likewise, the bender elements which are described as disc elements may be other than round if desired. Also,

scription of the invention in several embodiments there of is merely illustrative of the invention and is not to be taken as a limitation to the scope of the invention as clearly set forth in the claims which follow.

What is claimed is:

1. An underwater transducer comprising, first and second parallel spaced metal backing plates with broad inner and outer faces, a rigid metal coupling member mounted between said plates and attached thereto over a small fraction of the surface area of their opposed inner faces, first and second layers of piezoelectric material each with electrodes plated on the broad surfaces thereof, said broad surfaces being substantially the same shape and size as said broad faces, and means cementing one of said layers over each of said outer faces to form'a pair of flexural elements.

2. The transducer according to claim 1 wherein the layer of piezo-electric material on each backing plate has a thickness less than the thickness of said backing plate.

3. The transducer according to claim 1 including; a water-tight housing surrounding said flexural elements, said housing including a pair of resilient and sound transparent windows superimposed on said layer of piezo-electric material and a bathe support means connected between said housing and said coupling member to position said fiexural elements within said housing.

4. The transducer according to claim 3 wherein said baflie means includes at least one perforated metal sheet itis understood that the invention is useful for both transmitting and'receiving applications and that the operating frequency is not necessarily restricted to the low frequency range of the exemplary embodiments.

Furthermore, it is understood that the foregoing deextending parallel to said broad faces.

5. The transducer according to claim 3 wherein said housing member-is cylindrical, said flexural elements are disc shaped and said windows comprise the circular end walls of said cylindrical housing.

6. The transducer according to claim 5 wherein said coupling member interconnects the centers of said flexural elements.

7. The transducer according to claim 3 wherein said transducer includes a water-tight bellows connected within said housing, said bellows and said housing having apertures therein aligned to define a passageway from the exterior of the housing to the interior of the bellows.

8. The transducer according to claim 3 wherein at least two layers of piezo-electric material with electrodes plated thereon are cemented to and substantially cover said backing plate.

9. The transducer according to claim 8 wherein the layer of piezo-electric material on each'backing plate nearest said window comprises. substantially half the thickness of one of said flexural elements.

References Cited in the file of this patent UNITED STATES PATENTS 2,086,891 Bachmann et a1. July 13, 1937 2,242,757 Romanow May 20, 1941 2,448,365 Gillespie Aug. 31, 1948 2,636,134 Arons et a1. Apr. 21, 1953 3,002,179 Kuester Sept. 26, 1961 

1. AN UNDERWATER TRANSDUCER COMPRISING, FIRST AND SECOND PARALLEL SPACED METAL BACKING PLATES WITH BROAD INNER AND OUTER FACES, A RIGID METAL COUPLING MEMBER MOUNTED BETWEEN SAID PLATES AND ATTACHED THERETO OVER A SMALL FRACTION OF THE SURFACE AREA OF THEIR OPPOSED INNER FACES, FIRST AND SECOND LAYERS OF PIEZOELECTRIC MATERIAL EACH WITH ELECTRODES PLATED ON THE BOARD SURFACES THEREOF, SAID BROAD SURFACES BEING SUBSTANTIALLY THE SAME SHAPE AND SIZE AS SAID BROAD FACES, AND MEANS CEMENTING ONE OF SAID LAYERS OVER EACH OF SAID OUTER FACES TO FORM A PAIR OF FLEXURAL ELEMENTS. 